Molecules hitchhiking on comets may survive impacts
Posted: April 13, 2001

File image of comet. Photo: Courtesy NASA/JPL
Simulating a high-velocity comet collision with Earth, a team of scientists has shown that organic molecules hitchhiking aboard a comet could have survived such an impact and seeded life on this planet.

The results give credence to the theory that the raw materials for life came from space and were assembled on Earth into the ancestors of proteins and DNA.

"Our results suggest that the notion of organic compounds coming from outer space can't be ruled out because of the severity of the impact event," said research geologist Jennifer G. Blank of the Department of Earth and Planetary Science in the College of Letters & Science at the University of California, Berkeley.

Blank and her colleagues Randy Winans and Mike Ahrens of the Chemistry Division of Argonne National Laboratory, and engineer-mathematician Gregory Miller of the Applied Numerical Algorithms Group of Lawrence Berkeley National Laboratory, reported their preliminary findings at the national meeting of the American Chemical Society in San Diego, Calif. The talk was part of a session on extraterrestrial organic chemistry organized by Blank and colleague Max P. Bernstein, a chemist in the Astrochemistry Laboratory at NASA Ames Research Center in California.

Blank's team shot a soda-can sized bullet into a nickel-sized metal target containing a teardrop of water mixed with amino acids, the building blocks of proteins. More than seventy varieties of amino acids have been found in meteorites - many the suspected cores of comets that smashed to earth - and are presumed to exist in interstellar dust clouds.

Not only did a good fraction of the amino acids survive the simulated comet collision, but many polymerized into chains of two, three and four amino acids, so-called peptides. Peptides with longer chains are called polypeptides, while even longer ones are called proteins.

"The neat thing is that we got every possible combination of dipeptide, many tripeptides and some tetrapeptides," said Blank, a geochemist. "We saw variations in the ratios of peptides produced depending on the conditions of temperature, pressure and duration of the impact. This is the beginning of a new field of science."

Freezing the target to mimic an icy comet increased the survival rate of amino acids, she added.

The ballistic test was designed to simulate the type of impact that would have been frequent in Earth's early history, some four billion years ago, when rocky, icy debris in our solar system accreted to form the planets in what must have been spectacular collisions. Much of the debris would have resembled comets - dirty snowballs thought to be mostly slushy water surrounding a rocky core - slamming into Earth at velocities greater than 16 miles per second (25 km/sec).

The severity of the laboratory impact was akin to an oblique collision with the rocky surface of the Earth - a comet coming in at an angle of less than 25 degrees from the horizon, rather than head on perpendicular to the Earth's surface.

"At very low angles, we think that some water ice from the comet would remain intact as a liquid puddle concentrated with organic molecules," ideal for the development of life, Blank said. "This impact scenario provides the three ingredients believed necessary for life: liquid water, organic material and energy."

Benton C. Clark, chief scientist of Flight Systems at Lockheed Martin Astronautics, proposed in 1988 that if comets are slowed sufficiently, for example by drag from the Earth's atmosphere, some water and organic compounds might survive the collision. They would collect in what he called a "comet pond" of concentrated organic material where life could develop.

Though comet hunter Eugene Shoemaker estimated that in Earth's early history only a few percent of comets or asteroids arrived at low enough angles, the bombardment would have been heavy enough to deliver a significant amount of intact organic material and water, according to Blank's estimates.

The best known theory of the origin of life on Earth is that it derived from complex molecules such as amino acids and sugars produced early in the planet's history by electrical discharges in an atmosphere replete with gases such as methane, hydrogen, ammonia and water. The famous Miller-Urey experiment in 1953, conducted by Stanley Miller and Harold Urey of the University of Chicago, demonstrated that a lightening-like discharge in a test tube filled with these molecules could produce amino acids.

Other scientists, however, have proposed that the building blocks of life arrived from space. Astronomers have detected many kinds of organic molecules in space, floating in clouds of gas or bound up in dust particles. They range from the simplest - water, ammonia, methane, hydrogen cyanide and alcohols, including ethyl alcohol - to more complex molecules, including chains of up to eight carbon atoms.

Interestingly, of the more than 70 amino acids found in meteorites, only eight of them overlap with the group of 20 which occur commonly as structural components of proteins found in humans and all other life on Earth.

To test whether water and organic compounds could survive the high pressures and high temperatures of a collision, Blank and her colleagues worked for three years to design a steel capsule that would not rupture when hit with a mile-per-second (1.6 kilometer-per-second) bullet fired from an 80-mm bore cannon at the University of Chicago and later at Los Alamos National Laboratory. The target she and her team developed - a two-centimeter diameter stainless steel disk about a half-centimeter thick - was able to withstand about 200,000 times atmospheric pressure without bursting.

They filled the small cavity with water saturated with five amino acids: three from the list of 20 that comprise all proteins in humans (phenylalanine, proline and lysine) and two varieties detected in the Murchison meteorite (aminobutyric acid and nor-valine). That meteorite plummeted to the ground in 1969 in Australia and is thought to be the core of a comet.

The liquid contents were analyzed afterwards at Argonne using liquid chromatography and mass spectroscopy to determine the species and concentrations of molecules present.

The survival of a large fraction of the amino acids and their polymerization during the collision make the idea of an extraterrestrial origin of organic compounds a strong contender against the Miller-Urey theory, Blank said.

"About one comet per year arriving in a low-angle impact would bring in the equivalent of all the organics produced in a year in an oxidizing atmosphere by the Miller-Urey electric discharge mechanism," Blank estimated. "An advantage is you get all of it together in a puddle of water rather than diluted in the oceans."

The next hitchhikers she plans to subject to a shock test are bacterial spores, which some have proposed arrived on Earth via comet to jump-start evolution.

The work was sponsored by the National Science Foundation, NASA and the Department of Energy.